The present invention relates to methods for monitoring corrosion rates of mechanical components of fluid processing plants. As is well known in the art, a complex fluid processing plant such as an oil refinery or a chemical processing plant includes many different types of mechanical fluid containment components, such as piping, valves, pressure vessels, heat exchangers, fired heaters, atmospheric tankage, etc. Each such component will corrode at a specific rate that is a function of many variables including for example types of fluid within the component, fluid flow conditions, materials of construction, operating temperatures and pressures of fluids within the component, along with many other often complex factors.
Safe and efficient operation of such fluid processing plants requires that each mechanical component be inspected at regular intervals to monitor a rate of corrosion within the component so that the component may be replaced or taken out of service to avoid a corrosion-triggered breach of the component and a resulting failure of the plant, with potentially catastrophic results. Schedules generated for inspection and/or replacement of such mechanical components are generated based on optimizing safe and reliable plant operation while minimizing inspection and replacement costs. An important factor in generating such inspection schedules is a necessity of inspection personnel focusing attention on critical components having a high probability of compromising plant safety and reliability in the event of a corrosion caused breach of such a critical component. For example, scheduling inspection of all components more frequently than required to insure safe and reliable plant operation may actually divert inspection resources from the critical components, thereby over using inspection resources inefficiently while actually jeopardizing safe and reliable plant operation.
Many methods have been developed to generate efficient corrosion rate inspection schedules for fluid processing plants. Recent methods may include modern, computer-based organization and manipulation of large amounts of data related to specific inspection locations associated with all or exemplary components within the plant. Typical of such modern methods is the "Piping Corrosion Monitoring System Calculating Risk-Level Safety Factor Producing An Inspection Schedule" disclosed in U.S. Pat. No. 4,998,208 issued on Mar. 5, 1991 to Buhrow, et al. and assigned to the Standard Oil Company of Cleveland Ohio, which patent is hereby incorporated herein by reference.
The corrosion monitoring system of Buhrow et al. discloses a method including a first step of dividing a fluid containment plant into circuits wherein each circuit includes components made of similar materials exposed to common corrosive agents operating under similar operating conditions. In the next step individual inspection locations or points are defined within each circuit and historical corrosion rate data is assembled that includes actual thickness measurements at each inspection point, wherein the measurements are associated with specific times of measurement of the inspection point. Next, a highest rate of corrosion for each inspection point is determined based typically on analysis of a plurality of "test cases" which are established for each circuit. Some of the test cases are based on corrosion mechanisms that tend to corrode an entire section of pipe (e.g., to split a pipe), while other test cases are based on corrosion mechanisms tending to corrode specified points within a pipe or vessel (e.g., by pitting the component). The test case yielding the highest rate of corrosion for a specific inspection point is selected. Next a risk-level safety factor is established for each circuit and is calculated from a plurality of factors including operating pressures and temperatures of the circuit; relative degree of hazard to humans of the fluids in the circuit; potential of those materials to spontaneously ignite in the atmosphere; and the location of the circuit relative to valued property that could be damaged in light of a breach within the circuit. Finally, the risk-level safety factor is combined with the test-case generated rate of corrosion to produce an inspection date for each inspection point, and then an inspection schedule for the circuit and the plant is generated from the inspection dates.
As is apparent, the Buhrow et al. corrosion monitoring system is heavily dependent upon identification of a specific circuit. An exemplary circuit shown and described in that patent is identified as "a light gas-oil processing line" that includes many piping sections, valves, drains, reducers, vents, fittings interconnecting the components, etc. The circuit-based test cases that define a plurality of potential corrosion rates combined with the circuit-based risk-level safety factors essentially form a basis for the next inspection date for a particular inspection point. Additionally, after definition of the test cases, the Buhrow et al. method does not provide for any mechanism to re-define the circuits. Therefore, generation of shorter interval, higher frequency inspection schedules for the entire circuit is the primary result of inspections revealing higher corrosion rates. While such a method may be cautiously conservative, because the method is based on definition and test case analysis of static circuits it is inherently incapable of identifying many corrosion mechanisms and trends that may develop, particularly when such corrosion mechanisms and trends are localized within the circuit. In circuits where such localized conditions are present, that method may wastefully over-inspect some inspection points while dangerously under-inspecting other points.
In particular, the Buhrow et al. method establishes historical corrosion rate data based on an analysis of an assumed common corrosive environment even when the data reveals that such an environment does not exist in the circuit as defined. When analyzing corrosion rate data, it is of vital importance to recognize that process conditions, hence the corrosion behavior in a fluid processing system, may not observe the intended engineering/design criteria and ideal or expected fluid processing conditions. It is frequently observed that unanticipated conditions occur involving possibly flow rate changes, phase changes, particulate and/or fluid contaminants that change actual corrosion rate mechanisms within a so-called circuit. For example, an entrained particulate which forms in or is transported by a process fluid may dramatically erode internal surfaces of a change of direction fitting such as an elbow or tee fitting while having no discernable effect on the underlying fluid-generated corrosion rate on linear piping or storage containers. The Buhrow et al. method would rely on the conservative circuit-based test case averages and risk-level safety factor to have an adequately frequent inspection schedule to lead to replacement of the change of direction and other components in the circuit prior to breach, and would be unable to identify this primary corrosion mechanism at work in the circuit.
Additionally, the circuit-based test case calculations of corrosion rates in Buhrow et al. assume the corrosion rate data follows but one statistical distribution, namely a Gaussian or normal distribution. For example, the "circuit formula adjusted average rate" inflates the circuit average rate by a multiple of circuit corrosion rate standard deviations, with adjustments for the number of inspection points in the circuit, and thereby attempts to estimate a maximum circuit rate based on measurements of corrosion rates at individual inspection points. The calculation used therein is valid only when the corrosion rate data follows a normal distribution. However, recent work on fluid processing plants by the inventor of the "Method of Selective Corrosion Rate Analysis" invention described hereinbelow including over 240,000 inspection points in over 2,200 circuits has revealed that less than ten per cent (10%) of the circuits include corrosion rate data exhibiting a normal distribution. In circuits having sub-populations of corrosion rate data and those which may additionally exhibit non-normal corrosion rate behavior, using an inflated circuit average corrosion rate based on a normal distribution may result in over-inspection of points which follow a low corrosion rate distribution and under-inspection of sub-populations of points in a high corrosion rate distribution, so that none of the points in the system are optimally inspected.
The Buhrow et al. method therefore must select the highest test-case average corrosion rate, and then modify that rate by the risk-level safety factor to produce an inspection schedule because the static, rigid test-case comparative models cannot accurately describe all active corrosion mechanisms at work in the circuit. Neither Buhrow et al. nor any known methods of corrosion rate analysis of a fluid processing plant provides a model suitable for application of inspection data from which a corrosion inspection schedule may be generated for the plant that effectively quantifies and optimizes both inspection cost and probability of failure.
Accordingly, it is the general object of the present invention to provide a method of selective corrosion rate analysis for a fluid processing plant that overcomes problems of the prior art.
It is a more specific object to provide a method of selective corrosion rate analysis that enables a user to define a plurality of corrosion engineering models and select corrosion engineering models appropriate for the equipment and fluid(s) processed in the plant.
It is another specific object to provide a method of selective corrosion rate analysis that enables a user to identify multiple corrosion mechanisms at work in systems throughout the plant.
It is yet another specific object to provide a method of selective corrosion rate analysis that enables a user to generate a dynamic library of data that serves as a basis for modifying existing corrosion engineering models and developing new corrosion engineering models to enhance the analysis of subsequent inspection data.
It is another object to provide a method of selective corrosion rate analysis that enables a user to generate sub-sets of data groups within the plant for separate inspection schedules based upon identification and analysis of multiple corrosion mechanisms.
These and other objects and advantages of the present invention will become more readily understood when the following description is read in conjunction with the accompanying drawings.